The present invention relates to a preconditioner, a rectifier arranged to receive an AC voltage from a voltage supply and to rectify said AC voltage, a switch mode power supply arranged to control the current in the rectifier, and a feedback path for feedback of a preconditioner output voltage to a control block, said control block having a bandwidth equal to or greater than the voltage supply frequency. Such a preconditioner can advantageously be implemented in any electronic apparatus meeting the IEC61000-3-2 requirements, e.g. in a TV set.
To get a relatively constant DC output voltage from an oscillating (normally sinusoid) voltage source, this voltage is usually rectified with a rectifier bridge consisting of four diodes and a capacitor. Such a simple solution results in an AC line input current with a high harmonic content, as the peak current is high as the charge time of the capacitor is short. This also leads to undesired losses in the rectifier. Therefore, efforts have been made to generate a more harmonic AC line current. A preferred solution is to introduce a switch mode power supply (SMPS), and to arrange a suitable control of the SMPS. The input current to the SMPS can be sinusoidal (e.g. with the help of a Power Factor Controller), or trapezium (block shaped). This type of systems are used today as preconditioners in different electronic appliances.
It can be shown that in a rectifier system with an AC input voltage feeding a constant power load, the output voltage from the SMPS will always have a ripple with twice the frequency of the voltage frequency. When the voltage control loop, which is designed to keep the output voltage of the preconditioner at a predefined level during any changing conditions, tries to eliminate this ripple, the desired current shape is distorted, and extra harmonics occur in the input current. In order to avoid this drawback, the frequency bandwidth of the control loop can be limited to a very low value (below 50 Hz, typically 8-10 Hz), thereby eliminating the influence of the AC source in the system. With such a low bandwidth, the ripple in the output voltage is xe2x80x9cinvisiblexe2x80x9d to the control loop, and thus no attempt is made by the control loop to reduce the ripple. However, the low bandwidth leads to a very slow system characteristics, resulting in unsatisfying response to changes in the load impedance.
Therefore, a higher bandwidth is desired in the feedback loop, preferably higher than the frequency of the AC source, i.e. normally the mains frequency, e.g. 50-60 Hz. The disadvantage with this solution is as said that the ripple on the output voltage is fed back in the feedback loop and causes the input current to the SMPS to deviate from the ideal waveform.
The object of the invention is to provide a preconditioner with high bandwidth voltage feedback, that overcomes the above stated problem.
According to the invention, this and other objects are accomplished by a preconditioner of the kind stated by way of introduction, wherein the feedback path comprises a bandpass filter connected to said output voltage, and means for generating a feedback signal based on the output voltage and an output signal from said bandpass filter, which feedback signal has reduced ripple around the passband of the filter.
The bandpass filter operates to select voltage oscillations around the passband, and this output signal is then used to form a substantially ripple free control feedback. By performing the ripple reduction with the help of a bandpass filter, the elimination of ripple can be accomplished without sacrificing the bandwidth of the control loop, enabling a fast response to sudden load changes.
By eliminating the ripple in the voltage feedback loop, the preconditioner does not waste energy in an attempt to control this ripple, which is inherently present due to the AC source.
A preconditioner according to the invention may for example be used in LCD monitors requiring an input power larger than 75 W.
According to a preferred embodiment, the means for generating a feedback signal comprises a subtractor having a first input terminal connected to said output voltage, and a second input terminal connected to said bandpass filter output signal, for generating a feedback signal equal to the difference between said output voltage and said filter output signal.
According to another preferred embodiment, the bandpass filter is an inverting filter, and the means for generating a feedback signal comprises an adder having a first input terminal connected to said output voltage, and a second input terminal connected to said bandpass filter output signal, for generating a feedback signal equal to the sum of said output voltage and said filter output signal.
The bandpass filter preferably has a center frequency (xcfx890) substantially equal to twice the voltage supply frequency. This causes an efficient elimination of the ripple resulting from the AC source.
According to further preferred embodiment, the bandpass filter is comprised of two separate bandpass filters having different passbands, connected in parallel and their outputs summated together. This is advantageous in preconditioners distributed over large markets, including countries having different mains frequencies. Preferably, one passband lies in the interval 90-110 Hz, and another passband lies in the interval 110-130 Hz, in order to efficiently select ripple resulting from 50 Hz or 60 Hz mains frequencies.
The switch mode power supply can for example comprise a pulse width modulator.